1. Field of the Invention
The present invention relates to RFID (Radio Frequency Identification) or other non-contact type IC cards.
2. Description of the Related Art
Data carrier systems in which signal transmission/reception is conducted by causing an induced voltage to be generated in the antenna coil of a responder (non-contact type IC card) without any contact, by means of electromagnetic waves produced by an interrogator (reader/writer), are widespread.
The principal circuit element of these non-contact type IC cards comprises an IC chip, and a resonance circuit that comprises a capacitor and an antenna coil, as shown in FIG. 9. Accordingly in its essential structure, the antenna coil, capacitor and IC chip are disposed on a substrate.
Such antenna coils have conventionally taken the form of a thin metallic wire that is wound in the shape of a ring on the same surface. However, recent years have seen the increased use of an etched ring-shaped layer of copper foil or other conductive material that has been laminated on one side of the insulating substrate, in order to improve the antenna properties and mechanical strength as well as to reduce the number of steps required in manufacture.
In addition, polyimide films have been widely used as the insulating substrate of the laminated sheet that forms the antenna coil, or as a capacitor dielectric.
However, polyimide has a high water-absorption rate and its dielectric constant can be changed by absorbed moisture. It therefore creates problems, in capacitors in which polyimide films are used as dielectrics, that the electrostatic capacity is affected by moisture absorption and IC card resonance frequency deviates.
When the capacitor is formed from a laminated sheet in which the copper foil is bonded to the polyimide film using an adhesive in particular, the adhesive will, in addition to the polyimide film, also cause the dielectric constant to fluctuate, which results in a virtually unstable resonance frequency in the IC card.